1. Field of The Invention
The present invention is directed toward the fastener art and, more particularly, to a fastener of the type generally referred to as a "push-in" or "drive" fastener.
2. Description of the Prior Art
"Push-in" or "drive" fasteners of a variety of types and styles are well known in the art. Typically, they are arranged to be driven axially into preformed openings in a work piece and to engage the workpiece by virtue of laterally extending flexible tabs or wing-like members. The work piece may be a panel of a particular type, the fastener being used in the particular case to secure another panel, or strip of molding or the like, thereto.
Fasteners of this variety are often referred to as tree fasteners because the flexible tabs or wing-like members extend outwardly from a shank as branches do from the trunk of a tree.
Ongoing problems with such fasteners include the relationship between the installation or insertion forces and the removal forces. Ideally, it would be desirable in many situations to have a design wherein the insertion forces are extremely low and the removal forces extremely high. In many prior art designs, these forces are nearly equal. Attempts to reduce the forces resisting installation while increasing the forces resisting removal have typically involved changing the shape of the tabs, their angle of inclination, or their manner of connection to the fastener shank. These changes have generally been of marginal value.
Other problems include the difficulty in maintaining the fastener centered in a hole, so that it may have less of a tendency to wander out. Further, it has always been difficult to keep the tabs flexed inside the hole to discourage rattle and rotation without including an overdrive feature, such as a resiliently collapsible head in the shape of an umbrella.
The prior art is replete with tree-type fasteners. For illustration, U.S. Pat. No. 4,728,238 shows a one-piece drive fastener having a head and a shank, the shank including a generally non-circular cross section presenting a plurality of generally flat, axially extending surfaces. Each of the flat surfaces on the shank carry a plurality of axially spaced flexible resilient wing elements; the wings being flexed along the straight line connection with the flat surfaces. Certain wings extending from and positioned in spaced relation along at least one of said surfaces are angularly disposed so as to tilt in the direction of the head and further include means formed integrally with each of the certain wings to substantially increase the force required for removal of the fastener from a cooperating aperture as compared to the force required to introduce the fastener into the aperture. Nevertheless, this fastener is not self-centering, and tends to walk out of the holes when shifted back and forth. Forces required for its removal are inconsistent in value.
U.S. Pat. No. 4,776,739 shows a one-piece plastic reusable push-in fastener for insertion into an aperture in a workpiece comprising a head portion and a shank portion, the shank portion being integrally formed on one side of the head portion with its longitudinal axis extending away from the head portion and having a particular cross section, and a transversely extending fin section intermediate the length of the shank on one side thereof whereby the fin section cooperates with the particular shank cross section to inhibit withdrawal of the fastener from a particular workpiece aperture and to centrally align the longitudinal axis of the shank with the central axis of the aperture by forcing the periphery of the shank cross section against the inner wall of the workpiece aperture. While this fastener is self-centering, it makes minimal contact with the wall of the aperture. As a consequence, the fastener is able to rock in thin panels. The ratio between the forces required for removal and insertion is also very low, being close to 1:1.
In short, a tree-type fastener having the ability to remain centered in its hole, as well as the ability to remain flexed in the hole without the use of an overdrive, which is much more difficult to remove than to insert into the hole, has been long-sought in the art. Such a fastener is provided by the present invention.